nissan altima troubleshooting question?

[GimmeSomemore]

hi people. i own a 1999 nissan altima gxe sedan. this car runs awesome.. but i have one problem. the stupid check engine light is on. so i go get it checked out at autozone for free. the code is p0325 knock sensor and another for the emission test evap control, loose gas cap, evap purge etc. so when i go to the nissan dealer and spend about 550 bucks on a new evap canister and a evap purge and all the new houses and clamp. the check engine light goes off soon as i exit the nissan dealer. i go home change everything. seems fine for 3 weeks then bam stupid light just turned on again a couple days ago. so i go back to autozone and the same codes come up. people what the hell do i do. i know guessing is a faulty and very expensive stupid decission to do but i dont wanna spend the 90 bucks for the real dealer check and for them to say the same shit autozone has told me. the guy i had bought the car from had put a new exhaust in it. can that be the problem something with the weld maybe a pin hole in it? can it be the two evap canisters on the engine next to the firewall? or can it be just anything. the dealer said that there is now way that the knock sensor just comes on like that. so i know my issue is with the evap control somewhere in the car.. please help me?